The American Society for Reproductive Medicine (ASRM) has presented valid arguments against reproductive SCNT and concluded that it is unethical to use it as an assisted reproductive technology due to safety concerns, because of the developmental abnormalities seen in clones, and the undefined effect on children, relatives and humanity [140]. The technique of SCNT is presently performed in several laboratories worldwide to create human stem cells. strategies that can be applied to improve epigenetic reprogramming by SCNT; applications of SCNT; the ethical and legal implications of SCNT in humans; and specific lessons learned for establishing an optimized SCNT protocol using a mouse model. was initially exhibited in the mouse [9], with the aim Sitravatinib of creating and harvesting stem cells that could potentially be used to treat diseases. Subsequently, using comparable SCNT techniques, many species including cattle [10,11,12,13,14], mouse [15,16,17,18,19,20], pig [21,22,23,24,25,26,27], rabbit [28,29], rhesus macaque [30,31], and several more have been cloned successfully, producing viable offspring or ESCs for the purpose of reproductive or therapeutic cloning, respectively [32]. Despite previous achievements, SCNT remains an inefficient process; many abnormalities are seen in cloned animals and the overall efficiency of creating normal viable offspring in animals by SCNT varies, ranging between 5% and SPN 10% [33]. Blastocyst development in human oocytes after SCNT also varies; however, the realistic expected rate is usually 10% [34,35,36,37]. In this review, we begin by providing a brief overview of the abnormalities found in cloned animals, followed in more detail by SCNT protocol development, epigenetic reprogramming, applications, and the ethical and legal implications of SCNT in humans. All illustrative images were created by Sitravatinib C.G. using Microsoft Word 2016. 2. Abnormalities in Cloned Animals Reproductive cloning by SCNT with any donor cell type results in losses during pre- and post-implantation, as well as throughout pre- and post-natal development [33]. The first phenotype of clones is cell cycle arrest. The first defect in clones is genome instability, even before transcriptional abnormalities [38]. This shows that epigenetic processes involved in the differentiated state not only affect transcription, but also DNA replication. During development, cell-type-specific limitations in proliferation are an important component of cell differentiation. The barriers to reprogramming are genome instability first, and, second, transcriptional reprogramming. The first barrier is a requirement for the second. The developmental defects discussed below are all later in development. It is difficult to determine cause and consequence that late in development, as a primary defect leads to secondary consequences. Miscarriage and fetal mortality rates are high and frequently observed as a result of developmental defects in live clones produced from many species, and the latter has been attributed to incomplete reprogramming of the somatic nuclei by SCNT [39]. Insufficient remodelling Sitravatinib and reprogramming of the nucleus results in abnormal gene expression, subsequently contributing to abnormal placental and fetal development [40]. The latter has been called large offspring syndrome which is known for various phenotypes during pre- and post-natal development. During gestation, phenotypes such as hydroallantois, reduced mammary development and extended gestation have been observed [41]. Phenotypes noted at birth include large birth weight, abnormal organ size, motor control loss, enlarged tongue, and the development of respiratory problems as well as a weakened immune response in young clones soon after birth [42,43,44,45,46]. Obesity is an additional phenotype observed in adult clones [47]. However, species-specific differences do exist. At birth, bovine clones are more susceptible to obesity, whereas pig clones are underweight and have underdeveloped placentas [41,48]. Murine clones on the other hand have been associated with underdeveloped placentas in the early stages of gestation [49,50], but from the midpoint of gestation to birth there has been an association with placental hyperplasia [40,51,52]. In mice, abnormal epigenetic modifications including aberrant DNA methylation and histone modifications have been revealed in cloned embryos [53,54,55]. Furthermore, in addition to abnormal placentas [40,51], several abnormalities have been found in full-term murine offspring that have led to early death due to respiratory failure or other deformities [56,57], obesity [47], liver necrosis, tumours and pneumonia [58]. Several factors have contributed to the low efficiency of SCNT including invasive micromanipulation; oocyte incompetence and variation in.
